The invention pertains to the method and equipment for processing of organic materials containing nitrogen compounds, which consists in an anaerobic digestion of the organic material under liberation of biogas containing methane and carbon dioxide.
"Organic materials" means, in the present description, excrements of hogs and black cattle, waste from alimentary and chemical industry, city sewage sludges and various kinds of biomass which may be employed in the process of anaerobic digestion to obtain biogas.
Several methods have been recently employed in the processing technology of the above-mentioned organic materials. Some of them are only intended as a suitable physical or mechanical treatment, e.g. dewatering or drying to the form of fertilizer or a raw material for further processing. The composition of organic materials does not substantially change during such treatment. A part of the treatment sometimes includes the step of processing by heat, the purpose of which is to destroy harmful microorganisms, see German Patent Application(DOS) 520,287 and British Pat. No. 1,492,396.
Biological aerobic oxidation is also used in the liquification of these organic materials which is realized by means of several technological variants. The common problem of these technologies of the state of the art is that the valuable components and potential energy present in the processed organic material are not fully utilized; high start-up costs and operation expenses; and the sensitivity of the biological process to external influences. According to the German Patent Application (DOS) 2,723,906, excrements are sterilized by alkaline hydroxides prior to the aerobic oxidation to mitigate the afore-mentioned drawbacks. In addition, urea is partly transformed to ammonia and its compounds during this process. The treatment comprising aerobic oxidation has a high power consumption demand for operating the aeration equipment. Thus, 2 kWh are consumed for removal of 1 kg of the dry organic material. Another disadvantage lies in the production of very undesirable nitrates which wind up in the waste water.
Anaerobic digestion has been utilized a long time in the treatment of waste, above all in sewage plants. The process takes place in sealed heated tanks without leaking of offensive odor. Biogas containing aobut 60% CH.sub.4 and 35% CO.sub.2 is a byproduct and is formed in the amount of about 300-700 liters per kg of the dry organic material introduced into this process, depending on the material composition. The calorific value of gas is about 6000 kcal/Nm.sup.3. U.S. Pat. No. 4,057,401 describes a method which employs an additional heating of the digestion tank by solar radiation to intensify the heating process.
A fundamental disadvantage of the purification processes by anaerobic digestion of the state of the art is the insufficient utilization of all valuable compounds present in the organic materials and the high contamination of the digested product which has to be further purified, for example, biologically. Such post purification is very difficult with animal farm waste and is realized only by mixing with city sewage water to achieve high dilution. This condition requires locating large fattening stations near large cities, which is undesirable for other reasons. Similarly, as in the aerobic purification, also in the more recent procedures using anaerobic digestion, the main part of nitrogen, which is the most valuable component of waste, is disadvantageously transferred into solution in the form of ammonia and oxidized during the subsequent biological purification to nitrates. Nitrates remain in clear water as a very harmful component.